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Thinning Practices in Southern Pines - With Pest Management Recommendations

T. Evan Nebeker – Respectively, professor, Department of Entomology,
John D. Hodges – Professor, Department of Forestry, Mississippi State University, Mississippi State, MS,
Bob K. Karr – Assistant professor, Department of Forestry, Mississippi State University, Mississippi State, MS, and
David M. Moehring – Professor (deceased), Department of Forestry, Mississippi State University, Mississippi State, MS.

United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.

Management Recommendations to Reduce Losses

Minimizing Damaging Agents

Any thinning strategy must consider the potential hazards associated with intensive silvicultural practices. The following management practices are recommended to minimize the impact of damaging organisms and environmental factors on pine stands.

Southern pine beetle. Southern pine beetle infestations are often associated with poor tree vigor. Because tree vigor is basically related to site, tree, stand, and environmental conditions, the development of southern pine beetle outbreaks is strongly influenced by these conditions. Though vigor is difficult to quantify, radial growth rate can serve as a strong indicator of tree condition on vigor. Other factors that affect vigor include age, stand density, species composition, soil texture, and type, drainage patterns, and stand disturbances associated with cultural practices.

Poor tree vigor is usually associated with densely stocked stands and declining or slow radial growth, conditions readily alleviated by thinnings, especially those that remove the lower crown classes. These types of thinnings eliminate the less vigorous or weakened trees that are the prime targets of southern pine beetle attack. Reduced competition pressure enhances the vigor of residual trees. Thinning stands back to 70 to 100 square feet/acre basal area reduces the risk of attacks and may also help to slow spot growth if an attack does occur. For greater effectiveness, thinning is generally timed in winter when the beetle is least active. Thinning to reduce southern

Thinning densely stocked stand to
reduce threat of southern pine beetle.
pine beetle hazard is recommended when basal area approaches 120 square feet/acre or when live crown ratios drop to about 40 percent. A carefully carried out thinning will stimulate radial growth, reduce evapotranspiration, and increase rain throughfall. The reduction in evapotranspiration slows down the exhaustion of groundwater supply and favors continued diameter growth. The prevention of severe water stress results in lower concentrations of monoterpenes and higher levels of resin acids, which could be involved in making the stand less attractive to beetles (Hodges and Lorio 1975).

Pine stands in low-lying areas are frequently subjected to flooding and become attractive to southern pine beetle. In these areas, thinning alone may not correct the problem. Additional management actions such as drainage to divert excess water may be needed.

Any thinning strategy to reduce the risk of southern pine beetle attack should be compatible with management goals and consider such things as site and stand factors, equipment, seasonality, and product objective. Management of other potential hazard (e.g., annosus root rot, Ips spp., and black turpentine beetle) that might conflict with recommendations for southern pine beetle must also enter into the decisonmaking process.

Annosus root rot. Thinning is the single most important factor contributing to annosus root rot in pine stands, since cutting exposes stump surfaces to infection. Damage due to the fungus increases with time after thinning up to about 8 years, after which the damage level stabilizes. Because annosus spore production is at its highest level in January and February, thinning during the winter increases the likelihood of infection. In addition to thinning, species susceptibility, virulence of the disease, deep sandy soils, low soil organic matter, air temperatures below 70ºF, duration of stump susceptibility, and pruning contribute to and/or facilitate infection.

A comprehensive survey of annosus root rot damage in planted and natural stands throughout the South revealed that 2.8 and 0.07 percent, respectively, were infected. In scattered high-hazard areas, the 5-year loss in volume following thinning was estimated to be 20 percent of the stand (9 of 46 cords/acre). In general, volume loss following thinning ranges from 0.1 to 0.5 cord/acre/year (Alexander et al. 1981).

For high-harzard sites, the following measures are recommended for minimizing losses to annosus root rot (Kuhlman et al. 1976):

  • Delay thinning or reduce the number of thinnings to reduce the risk of loss. Wider spacing and reduced thinning are beneficial practices.
  • Use borax on cut stumps for the most positive control. Borax is not effective at a second thinning if not used for the first.
  • Thin from April to August south of 34°N latitude to provide passive control because of high air and stump temperatures (which are lethal to disease spores) and low number of spores.
  • Don't take any special precautions when replanting previously infected sites. The disease does not persist in the soil.
  • Plant more resistant species on high-hazard sites, e.g., longleaf pine is more resistant than loblolly pine.
Treating stump with borax to
control annosus root rot.

There is some evidence that prescribed burning will reduce the severity of annosus root rot in thinned plantations (Froelich et al. 1978). On low-hazard sites, chemical treatment of the stumps is of doubtful value (Hodges 1974). It is generally believed that on sites rated low hazard for annosus root rot, no restrictions on thinning are necessary. Although the best strategy for reducing the disease on high-hazard sites may be to delay or do no thinning, stands on low-hazard sites may be thinned based on normal silvicultural prescriptions dictated by product objectives, biological constraints, and desired capital recovery. As recommended above, stumps on high-hazard sites should be treated with borax and, in stands with confirmed root rot, Peniophora gigantea (Fr.) Massee, a saprophytic fungus, and, when possible, thinning should be done during the hottest months of the year (May-August) to take advantage of high temperatures and low spore production and survival conditions. Prescribed burning may be done before and after thinning to further insure the protection of residual stands from infection.

If spacing is wider than 8 by 8 feet, and the product objective is pulpwood, thinning may be foregone, particularly on high-hazard sites. Chemical thinning should be done for precommercial thinning on high-hazard sites.

Because thinning to reduce the hazard of southern pine beetle incidence conflicts with management recommendations for annosus root rot, foresters should be aware of the tradeoffs in areas where both pests are likely to occur. Benefits must be weighed against potential losses for any chosen thinning strategy. In most cases, thinning should be done in the winter to reduce the hazard of southern pine beetle infestation, and the stumps should be treated with borax to prevent annosus infection.

Fusiform rust. Losses due to fusiform rust have been estimated to exceed $25 million annually in value, making it the most economically damaging disease of southern pines. Slash and loblolly pines are the preferred hosts, slash pine being the more seriously affected. The disease is more severe in plantations than in natural stands, with mortality occurring primarily in the seedling stage.

Interestingly enough, cultural practices that favor fast growth of stands increase the incidence of fusiform rust. However, to prescribe against cultural practices that improve growth is not silvicultually and economically sound. It has been claimed that even a 50-percent rust infection rate in a stand can be offset by an increase in volume resulting from such intensive cultural practices as site preparation and fertilization.

Thinning has little or no practical value of reducing the incidence of fusiform rust because infection occurs at the early stages of stand development. It must, therefore, be practiced for a different purpose – to minimize losses due to rust, i.e., salvage. The first 5 years after planting are the critial period. Precommercial thinning may not be justified and may aggravate the problem by increasing the surface area for infection and by preventing natural pruning. This implication is supported by the finding that close spacing reduces fusiform rust incidence. Heavy thinning may also have an adverse effect by favoring the growth of alternate hosts (oaks), thereby enhancing rust incidence. Thinning of heavily infected stands, on the other hand, can profoundly affect total wood production if heavily infected trees certain to die before final harvest are removed. If rust incidence is less than 25 percent, the first thinning should remove the majority of the diseased trees. Opening up the stand too much can have unfavorable consequences on the residual stand in terms of growth and damage from ice and wind.

Stand with rust-infected trees before sanitation
salvage to remove severely infected trees.
Stand with rust-infected trees after sanitation
salvage to remove severely infected trees.

Wind/windthrow. Wind and windthrow are natural phenomena that cause extensive damage to southern pine stands. The severity of damage depends on geographic location, wind gustiness, and other factors.

In a number of studies, thinning influenced the amount of damage due to wind and windthrow. The heavier the thinning, the greater the wind damage. More crown damage (limbs and small branches broken off, needles and bark whipped off) occurred in heavily thinned stands. Although thinning in general increases wind and windthrow damage, it can potentially reduce such damage by removing diseased, high-risk trees. Because bigger trees are more prone to windthrow, an early thinning will improve the stability of stands after the remaining trees have adapted to greater exposure.

The formulation of a thinning strategy within the Coastal Plain hurricane belt should take into account the possibility of windthrow damage. The following considerations could help in developing an optimum thinning strategy:

  • Trees infected with annosus root rot and fusiform rust are prone to wind damage.
  • Shallow root systems favor windthrow.
  • Edge trees are more stable than interior trees.
  • Trees on soils extensively saturated with water are prone to windthrow.
  • Stand density and height alter the wind profiles.
  • Wind is funneled through gaps and saddles in main ridges, resulting in greater wind acceleration.
  • Indentations in the edge of stands, especially V-shaped openings, produce a funneling effect.
  • Logging injuries contribute to windthrow.
  • Windfall losses are heavy following partial cutting.

Ice/glaze. Slash, longleaf, and loblolly pines are generally more susceptible than shortleaf pine to glaze damage. Glaze damage can be very serious, depending on species, geographic location, age of trees, amount of ice formed on trees, stand density, presence of disease, crown characteristics, and diameter/height ratio. Thinning has very profound effects in modifying the degree of glaze damage. Studies have shown that increasing thinning intensity causes increasing amounts of glaze damage.

Glaze damage can be minimized by early manipulation of the growing space (precommercial thinning) to develop trees with sturdy, compact crowns. Adequate stocking must be maintained to provide mutual support among trees (Brender and Romancier 1965; Lemon 1961). In ice storm belts, loblolly should be thinned lightly (no more than one-third of basal area at a time) and more frequently (from below or selectively). If selective thinning is not feasible nor practical, row thinning at wider intervals (say every eighth or tenth row) with selective thinning within leave rows would be a desirable alternative (Shepard 1975). Selective thinning should remove the smaller, weaker trees. Bent trees should be pruned (Williston 1974).

Minimizing Felling Injuries

The following practices are suggested to limit damage to pines resulting from felling:

  1. Thin in winter and late summer because trees are more severely injured during spring and early summer. (See #7 below.)
  2. Continue to salvage high-risk trees after each cutting (or target for next thinning) to reduce infection courts.
  3. Mark leave trees, instead of those to be cut, because this calls attention to the crop trees.
  4. Use directional felling wherever possible.
  5. Use smaller machines to minimize unacceptable damage to residual trees.
  6. Establish stands at wider spacings to reduce thinning frequency.
  7. Because damage is reduced where there is frozen soil and snow cover and little sap flow, thin at the end of autumn and the beginning of winter, the best months in more northerly areas.
  8. Time operations so as to avoid wet weather logging to minimize stand productivity losses associated with soil compaction (Moehring and Rawls 1970).

Minimizing Skidding Injuries

Skidding-related damage can be reduced by adhering to the following practices:

  • On clay soils or soils saturated with moisture use skidding equipment that minimizes soil compaction; otherwise schedule skidding during dry weather (Moehring and Rawls 1970).
  • In stands that will tolerate little damage, consider using horses for skidding logs.
  • Cut logs short enough to minimize scarring of residual trees during forwarding operations.
  • Avoid damage to low-lying areas with fine textured soils by shifting logging operations to better-drained, sandy soils during wet periods (Hatchell et al. 1970).
  • Use smaller equipment to reduce the impact on soils and residual trees where the trails are dispersed. With larger equipment, it would be better to concentrate the impact on as few trails as possible because heavy equipment affects the soil to a greater extent during the first few trips.
  • Use cultural techniques such as ripping for rehabilitating damaged areas.
  • Hasten site recovery by loosening, revegetating, or mulching the disturbed area.

It has been suggested that 40 years are required for natural forces to bring soil conditions in loblolly pine stands of the southeastern United States back to normal (Hatchell and Ralston 1971). With regard to ameliorative conditions and restoration, the presence of logging residues after delimbing contributes to the prevention of significant compaction (King and Haines 1979). Disking, ripping, and subsoiling also ameliorate compacted soil conditions (Hatchell et al. 1970; Moehring 1970; Peters 1977).

Bedding (or a moderate amount of fertilizer) has been shown to improve growth of loblolly pine on compacted soils with a greater growth response obtained than on uncompacted soil. This has been attributed to elimination of competing vegetation by skidding (Hatchell 1981). Other biotic factors important in natural recovery of compacted soil include increased percolation rates and noncapillary pore space attributable to deep-rooted species like kudzu and alfalfa (Uhland 1950).

Dense layers of herbage aid in preventing soil compaction, just as does slash placed on the skid roads and over areas of heavier traffic. Increased herbage also reduces the amount of erosion, rain impact, vehicle impact, and high soil temperatures resulting from direct solar radiation.

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